RU2432224C2 - Method of producing gas turbine engine hollow vane ceramic cores - Google Patents

Abstract

FIELD: process engineering.

SUBSTANCE: invention relates to metallurgy, particularly, to production of vane core with at least one thin zone with thickness "e" located, in particular, on gas turbine vane rear edge. Proposed method comprises moulding mix made up of filler of ceramic particles and organic binder. Core is molded with thin zone beaded relative to thickness "e" by some surplus thickness E. core withdrawn from mould, binder is removed to cure core by thermal treatment. Surplus thickness is remove either by milling, or chipping core prior to hardening, or by grinding precalcinated core.

EFFECT: reduced number of defects and production costs.

10 cl, 9 dwg

Description

The present invention relates to a process for the manufacture of parts, for example, metal blades of gas turbine engines, having internal cavities of complex geometry, forming, in particular, cooling circuits, in accordance with the casting technology for lost wax castings.

The manufacturing process of such blades passes through the implementation phase of a model made of wax or similar material, which contains some internal part forming a casting core and determining the shape of the internal cavities of the blades. To form such a model, a die casting mold for wax is used, into which the core is placed and then molten wax is introduced into it. After that, the wax model thus obtained is subjected to rapid cooling several times in ceramic masses, which are a suspension of ceramic particles and intended to form a shell mold. Then the wax is removed and the resulting shell mold is fired. The finished blade is obtained by pouring molten metal, which occupies all the cavities between the inner wall of the shell mold and the core. Due to the crystallization nucleus or the corresponding selector and controlled cooling, the metal solidifies in accordance with the desired structure. In accordance with the characteristics of the alloy used and the expected properties of the casting part, directional curing of the metal in a column structure (DS), directional curing of the metal in a single crystal structure (SC) or equiaxial curing (EX), respectively, can be carried out. The first two families of parts relate to heat-resistant alloys intended for the manufacture of parts subjected to powerful thermal and mechanical stresses in a turbojet engine, such as, for example, HP turbine blades.

After the alloy has cured, the shell mold and core are removed. The result is a blade of the desired shape.

Used cores for molds are made of ceramic material, usually having a porous structure. These cores are made of a mixture formed by a heat-resistant filler in the form of small particles and a more or less complex organic component forming a binder material. Examples of such compositions are given in patents EP 328452, FR 2371257 or FR 1785836. As is known, foundry cores are made by casting using, for example, injection molding technology. This molding operation is followed by the binder removal operation, during which the organic fraction of the core is eliminated using one method or another, such as, for example, sublimation or thermal destruction, depending on the nature of the materials used. As a result of these operations, a porous structure is obtained. The core is then consolidated by heat treatment in an oven. If necessary, the final finishing step is carried out, designed to eliminate burrs and remove traces of the connection planes, as well as to obtain the required core geometry. To do this, use abrasive tools. It may also be necessary to reinforce the core so that it does not become damaged during subsequent cycles of its use. In this case, the core is impregnated with an organic resin.

In order to reduce the length of the manufacturing cycle of the cores, it is also possible to produce a blank of such a core and then mechanically process the slots and partitions in the case when the core is in an unprocessed state. This process is described in the patent application filed in the name of the Applicant under the number FR 0452789.

The geometry of the cores is always quite complex, in particular, the walls of some zones are always thinner. As a result, filling limits are often reached and the development of more fluid pastes or the use of more significant pressure is required to ensure that all concave mold elements are filled.

Thick cores are more dimensionally stable due to the composition of the pastes used. In this case, for example, the ratio between the content of the binder and the filler is selected, as well as the proportion of small and large ceramic particles of this filler.

Thus, in the framework of newly developed engines and engines that are commercially available, the injection molding method known from the prior art does not allow a sufficiently economical way to respond to changes in the design of casting cores, in particular, if necessary, to reduce the thickness of thin zones, the thickness of which has a value less than 0.4 mm.

In order to overcome the above problems, it is proposed in known technical solutions to produce ceramic cores in a mold, using which thin zones and / or critical zones are obtained either by using more fluid ceramic pastes or by modifying injection molding parameters, in particular flow rates or pressures having a value exceeding the conditions of traditional use. However, this technology presents certain limitations. On the one hand, the ceramic material has abrasive properties and the slice resulting from the new filling conditions is the reason for premature wear of thin zones of technological equipment. This entails numerous periods of production shutdown and an increase in the cost of maintaining this process equipment in good condition. On the other hand, despite the optimization of the filling conditions and despite the use of digital modeling, some thin zones freeze the filling front. It follows from this that such filling can be carried out only by gluing the so-called “cold” paste, that is, a paste whose temperature is not optimal in order to have a solid bond. These filling conditions are a source of signs such as cracks, which entail the rejection of a significant number of cores after they are expelled and the quality control of these cores. Such defects can also be detected after heat treatment, removal of the binder and calcination, which is even more damaging.

The present invention is to eliminate the disadvantages inherent in the known solutions. The problem is solved by a method of manufacturing a casting core containing at least one thin zone or thin wall having a thickness of "e", the value of which is in the range from 0.1 mm to 0.5 mm, for example, at the trailing edge of the blades of a gas turbine engine, including a step of molding in a mold a mixture having a filler composed of ceramic particles and some organic binder, a step of removing from the mold and a step of releasing the binder and heat treatment to consolidate the core. This method is characterized in that a core is formed in said mold, in which said zone is thickened with respect to the thickness "e" by some excess thickness E, and mechanical processing of said excess thickness is carried out after removing the core from the mold up to obtaining said thickness "e" in such a way as to create an opening channel sufficient for the flow of said mixture during its introduction into the mold. Moreover, the above-mentioned machining operation can be implemented both before and after heat treatment.

While specialists in the art will seek to develop materials with reduced viscosity or modify the parameters of the injection molding process, in particular flow or pressure, the invention is the result of another approach based on the reduction of pressure losses associated with the determination of the cavity to be filled .

The pressure loss can be expressed by the following relation: P = η · Q · L / π · D4, which relates pressure (P) to viscosity (η), flow rate (Q), length (L) and diameter (D).

In accordance with the invention, they affect the diameter of the passage in a narrow zone, increasing it so as to create a hole sufficient for the flow of the paste.

Thus, any specific distribution is overcome, even with a decrease in wall thickness of up to 0.1 mm.

Thanks to the invention, the cost of manufacturing casting cores is reduced. Despite the fact that the number of cores having signs such as cracks during injection molding and / or during calcination and obtained as a result of injection molding in a mold with a thin trailing edge reaches several tens of percent, the proposed technical solution allows to provide a significant gain in quality and to obtain cores having thinner trailing edges than when using the method known from the prior art. In this case, the intended limit drops to a thickness of 0.1 mm.

Preferably, the machining of a thick core zone is carried out mechanically by milling, although it can also be done manually.

More specifically, the core contains from 80% to 85% of a mineral filler and from 15% to 20% of an organic binder. Preferably, the composition corresponds to one of the compositions described in patent EP 328452, issued in the name of the Applicant. At the same time, a low-fluid composition is sought, which should give a relatively small change in shrinkage in the serial production of cores.

Thus, the present invention allows the formation of a single paste for the manufacture of a variety of different cores for the blades, while the method of the existing prior art requires the formation of adapted for each case pastes. In particular, fluid pastes must be provided for cores designed with trailing edges for which the thickness is less than 0.4 mm.

It is advisable that the machining is carried out by sequentially passing the cutting tool, removing in each pass a certain part of the excess thickness of the material, having a value in the range from 0.05 mm to 2 mm. In particular, before annealing, the machining is carried out using a cutter by removing material, whereas after annealing, machining is carried out using a tool, often a diamond, by removing material on a milling machine with at least three axes and preferably with four or five axes. With this tool, the automatic implementation of machining can be ensured.

This technology allows you to process an unburnt core based on an existing CFAO card (design and manufacture using a computer) without the negative effect of core shrinkage during the calcination process, which is not always identical. The unbaked core has the dimensions of the mold in which it was made. Preferably, the cores before calcination are geometrically identical.

In this way, molds with a variable thickness are realized corresponding to various structural elements of the core. Other forms are also possible.

Other characteristics and advantages of the invention will be disclosed in the description of an embodiment of the invention, given with reference to the figures, including:

figure 1 is a schematic sectional view of a cooled turbine blade;

figure 2 is a General schematic view of the core of the cooled blade;

figure 3 - schematic view of the zone of the trailing edge of the core, representing the excess thickness in accordance with the invention;

figure 4 is a schematic view of a portion of the trailing edge of the core after machining its excess thickness;

5 illustrates on a graph the change in casting pressure as a function of the techniques used to obtain the desired trailing edge geometry;

6 is a filling mold in the function of the techniques presented in figure 5;

7 is a method of machining using a mill;

Fig.8 is a section along the line 8-8 of the first spike shown in Fig.3;

Fig.9 is a section along the line 9-9 of the first spike shown in Fig.8.

The following description discloses an embodiment of molding a foundry core for manufacturing a high pressure turbine blade in a gas turbine engine used in aviation or in ground conditions. This option is not restrictive.

As shown in FIG. 1, the turbine blade 1 comprises a lower surface IN, an upper surface EX, a leading edge BA and a trailing edge BF. In the case when it comes to the blade of a high-pressure turbine of a gas turbine engine intended for use in aviation, this blade contains seven internal cavities, which are indicated by positions 1A through 1G. The trailing edge of the blade contains an opening 1H extending parallel to this trailing edge. This opening is fed from the last 1G cavity using a plurality of calibrated 1GH channels parallel to each other to eject a cooling fluid, which is the air taken from the compressor.

The said cavities are separated from each other by partitions, indicated by the positions 1AB, 1BC, etc. In the case of the manufacture of blades by casting molten metal, it is necessary to embed a core in the shell mold that occupies the voids corresponding to the cavities in the blade to be molded. This core, as follows from the drawing shown in figure 1, has a complex shape.

2, a core 100 obtained in a mold can be seen. This core contains a part corresponding to the cavities of the blade vane 100A, a part 100B corresponding to the cavities of the root part of the scapula, and a part 100C forming a handle for gripping the core during its manufacturing. In the head part of the blade vane, you can also see part 100D, denoted by the term baignoire ("bath") adopted in the art.

The trailing edge of the core, or the part indicated by 100H and forming the cavity 1H shown in FIG. 1, and the spikes 100GH forming the channels 1GH shown in FIG. 1, are shown in FIG. 3 or FIG. 4. A specific implementation of the first spike 100GH1 in accordance with the invention will be discussed in more detail below.

This core is made by injection molding in a mold in which the filling of thin zones formed by spikes 100GH should be ensured. In this case, the commonly used technology consists in the development of a mold with side parts that have a certain mobility in order to be able to remove the core after the injection of material under pressure into the mold and hardening it. As mentioned above, the introduction of material into these zones is all the more difficult the thinner these zones are.

The object of the invention is the implementation of a core having such a rather complicated structure, without the need to develop more fluid pastes or to increase injection molding parameters, such as pressure or flow.

In accordance with the invention, a modified mold is made, that is, a mold whose core after molding has at least one thin zone that is thickened.

The thickened thin zone of the first spike 100GH1 is obtained by forming a mold in this place to obtain such a thickened zone for the first spike 100GH1. This first tenon is the first in order when viewed from the root of the scapula, from where the paste forming the core is introduced under pressure. This part is shown in section in FIG. 8 and 9. In FIG. 8, the excess thickness E of the spike 100GH1 with respect to the upper surface 100Ex of the core 100 can be seen. The surfaces of the parts 100G and 100H on the outside of the outer surface of the blade are essentially in the same plane, except for this excess thickness. This excess thickness is determined as a function of the final thickness "e", which is desirable to obtain for the spike 100GH1, and the amount of paste that is introduced into the mold. Here we are talking about creating a channel with an opening sufficient for the flow of the paste during injection molding. In the cross section shown in Fig.9, the contour of the thickening E takes into account the rounded edges of the tenon. The formation of the radii of the rounded edges of the spike can also be carried out using machining.

The paste used preferably contains an organic binder combined with a mineral filler. So, for example, this mixture is implemented in accordance with the information from patent application EP 328452. The core has satisfactory strength when manipulating it with hands, and its structure allows you to work with it using a milling tool for chip removal or abrasive processing.

After manufacturing a core with a thickening E on the first tenon, the next step consists in machining one or more thickened zones on this core blank. This machining is preferably realized by means of a tool such as that shown schematically in FIG. 7. This is a milling cutter 200 comprising a cutting end 200A and a helical portion or a helical cutting edge located along its shaft 200B. This cutter is moved perpendicular to the surface to be treated. The rotation speed of the cutting tool, as well as the speed of its movement are fixed. In this way, the forces acting on the material are limited and the tool is not bent.

It is preferable to use a numerically controlled milling machine with five axes of movement, for example, with three axes for positioning the cutter in space and two axes for positioning the core. Such a machine can be easily programmed in order to automate, if necessary, the machining of the recesses.

In Fig.4, you can see the area of the trailing edge of the core after it has been machined. The channels have dimensional parameters, in particular, the thickness that they will form, up to shrinkage, in the part when the molten metal is poured into the shell mold.

After the core has been processed before calcining, they proceed to the subsequent processing operations, which themselves are known from the process of manufacturing the cores in the foundry, namely, the elimination of bonds, that is, the removal of the organic binder. To this end, the core is heated to a temperature sufficient to destroy the organic components that this core contains. Other steps consist in the subsequent heating of this core to the sintering temperature of the ceramic particles constituting this core. In the case when additional consolidation is necessary, the core is impregnated with an organic resin.

For machined cores, after calcining, they go directly to the final finish and to the control.

In order to demonstrate the benefits of the proposed technical solution, comparative tests were performed, the results of which are shown in FIG. 5 and 6.

On figa presents the filling phase of the mold from the prior art, shown by shaded lines. The thickness of the channels for forming studs in this embodiment is 0.35 mm. Here you can see that the paste is introduced through the zone of the root of the scapula and gradually spreads in the direction of the head of the mold. In this case, the paste is inhibited in its flow through the zone of small thickness. It even cools before passing through these zones. Thus, the paste should go around these areas. From this it follows that at the moment of contact between the two fronts of propagation, this paste is not fluid enough to form a reliable adhesion.

In the graph shown in FIG. 5, it can be seen that the required pressure is 94 units of measure for this pressure.

As can be seen in Fig.6b, the channel 60 is made from the side of the zone 100H, designed to power the paste directly. Indeed, the pressure due to the introduction of the paste is somewhat lower, that is, there is enough pressure, amounting to only 85 units of measurement of this pressure. However, here the spike is still not quite satisfactory, since the front of the paste remains fixed in the spike channels.

As can be seen in FIG. 6c, a false spike 70 is added. The result is essentially the same as in the previous case. Pressure, in this case, is 85 units of measure for this pressure.

As can be seen in FIG. 6d, a recess is made in the mold so as to form a bulge on the first tenon in accordance with the invention. Referring again to FIG. 5, it can be seen that the paste injection pressure of 78 units of this pressure is sufficient to prevent the paste propagation front from being blocked in this channel. This ensures that the trailing edge zone is filled through the said channels. It follows that no mechanical weakening will affect the channel area.

Here, the thickening of the first tenon of the core is mainly represented, but the same principle can be applied to all tenons. This technology allows, therefore, in a more general sense, to ensure the implementation of those parts of the core that are very thin and have a small width, as is the case for the part of the core located in the immediate vicinity of the trailing edge and containing channels for air passage, blades ejected from the inside in the final part of the cooling circuit and injected into the gas stream. However, this technology can be extended to the machining of any part of the core for which the same problem of freedom of flow can arise.

Claims (10)

1. A method of manufacturing a core (100) for casting blades of a gas turbine engine containing at least one thin zone having a thickness "e", the value of which is in the range from 0.1 mm to 0.5 mm, and located, in particular, on the trailing edge of the blades of a gas turbine engine, including a step of molding in a mold a mixture having a filler composed of ceramic particles and an organic binder, a step of removing the core from the mold, a step of releasing the binder and a heat treatment step, p designed for core consolidation, characterized in that in said casting mold an opening channel is formed that is sufficient for said mixture to flow during its injection under pressure into the casting mold, wherein the channel forms a thin core zone, said thin zone being thickened with respect to the thickness "e" to some excess thickness E, while providing mechanical processing of the said excess thickness after removing the core from the mold. 2. The method according to claim 1, characterized in that the machining is carried out before performing the heat treatment operation. 3. The method according to claim 2, characterized in that the machining of excess thickness is carried out mechanically by milling with chip removal. 4. The method according to claim 1, characterized in that the machining is carried out after the operation of heat treatment. 5. The method according to claim 4, characterized in that the machining of excess thickness is carried out mechanically using abrasive treatment. 6. The method according to claim 5, characterized in that the machining is carried out using a cutter by removing material on a milling machine with at least three axes of movement and preferably with four or five axes. 7. The method according to claim 1, characterized in that the zone having a thickness of "e" is located in the immediate vicinity of the trailing edge and forms a spike (100GH) of formation of a channel for removing internal cooling air from the blades of a gas turbine engine. 8. The method according to claim 7, characterized in that said tenon is a first tenon when viewed from a feeding point with a paste intended to fill a mold. 9. The method according to claim 7, characterized in that during the machining process, radii of curvature of the surface of said tenon (100GH) are formed. 10. The method according to claim 1, characterized in that a core is made comprising a plurality of said thin zones, said thickening being applied to a plurality of these thin zones.

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